1. Field of the Invention
This invention relates to a process. More specifically, this invention involves an improved process for the synthesis of cis-1,4 poly(isoprene).
2. Description of the Prior Art
The synthesis of cis-1,4 poly(isoprene) and the problems related to its preparation have been extensively documented in the open technical literature. The preferred technique for the preparation of this polymer is the initiation of polymerization of its corresponding monomer with a Ziegler/Natta coordination type catalyst mixture. This type of catalyst is responsible for the stereoregularity and high yield of the resultant polymer. Where the isoprene monomer used in preparation of the stereoregular polymer is obtained from hydrocarbon conversion processes (e.g. the cracking or dehydrohalogenation of hydrocarbons) extensive purification of the monomer is generally required to remove polymerization catalyst deactivating agents (e.g. .alpha.-acetylenes and cyclopentadienes). The failure to remove such impurities can substantially increase the amount of catalyst required to effectively polymerize the isoprene monomer. The removal of cyclopentadiene impurities from crude isoprene monomer can reportedly be achieved by treating the crude isoprene with maleic anhydride, see U.S. Pat. No. 2,935,540. Alpha-acetylene contamination of isoprene can also cause deactivation of polymerization catalyst, however, to a lesser extent than cyclopentadiene. Alpha-acetylene purification of the crude isoprene monomers can reportedly be achieved through the use of molecular sieves, see U.S. Pat. No. 2,900,430. Another technique for removal of .alpha.-acetylene contaminants from crude isoprene monomers involves contacting the crude isoprene monomer with metallic sodium in a hydrocarbon diluent under the appropriate conditions, followed by separation of the isoprene from the dispersed particulates (unreacted sodium and sodium reaction products) with a molecular sieve, see U.S. Pat. Nos. 2,935,540; and 3,285,989. It has also been suggested that .alpha.-acetylene impurity levels can be effectively reduced by pre-reacting the impure isoprene monomer with one or more of the components of the polymerization catalyst, U.S. Pat. No. 3,442,878 (Col. 1, lines 29-35). This technique has, however, proven unsatisfactory because the catalyst residue remaining in the monomer subsequent to such treatment is disruptive of polymerization of the treated monomer, U.S. Pat. No. 3,442,878 (Col. 1, lines 35-50).
The preparation of crude isoprene monomer from hydrocarbon conversion processes can also result in saturation of the resultant monomer product with water. U.S. Pat. No. 2,905,659 discloses that the presence of water in the monomer charge can result in deactivation of the polymerization catalyst (Col. 1, lines 51 through 55); and, in practice water is routinely extracted from the monomer prior to its introduction into the polymerization medium.
In continuous processes for the polymerization of crude isoprene to cis-1,4 poly(isoprene), only a portion of the isoprene monomer is ordinarily converted to polymer, the unconverted portion then being recycled back into the polymerization stream. It is often desirable, and under certain conditions essential, to maintain the concentration of monomer in the polymerization medium within certain predetermined limits. Thus, the polymerization is carefully monitored and new monomer continuously added to the process to replace that which has been converted to polymer. The replacement portion of the monomer (hereinafter referred to as "make-up" isoprene monomer) has traditionally been introduced into the process stream at a point remote from the polymer product, cis-1,4 poly(isoprene). The "make-up" isoprene monomer and the isoprene monomer with associated solvents (which is recycled back into the system from the polymerization reactors) are generally combined, dehydrated (e.g. as by distillation) and metered into a downstream reactor where they are combined with an appropriate amount of polymerization catalyst. The presence of contaminants in the "make-up" isoprene can, as indicated previously, result in deactivation of the polymerization catalyst and thus it is necessary to either (a) add additional amounts of catalyst to compensate for such anticipated deactivation or (b) subject the "make-up" isoprene to a further purification whereby the relative concentration of contaminants therein is reduced to acceptable levels. As is thus apparent, the manufacture of cis-1,4 poly(isoprene) rubber by continuous polymerization processes is inefficient, regarding its consumption of catalyst, since it is economically impractical to subject the "make-up" isoprene monomer to extensive purification prior to introduction into the polymerization stream.